Permafrost of the Oka Plateau (Eastern Sayan Ridge)

This article presents the results of the first comprehensive study of permafrost and frost mounds in the Sentsa River valley on the Oka Plateau (Eastern Sayan Ridge). The lacustrine‐alluvial deposits are represented by clayey silts and silts with visible ice contents up to 90%. Ground ice mainly comprises bicarbonate calcium or bicarbonate magnesium, enriched in ammonium, aluminium, silicon, phosphorus, iron and zinc. Rare earth element content varies from 0.010 to 0.500 μg/L. The distribution of δ18O and δ2H in the ground ice to a depth of 20 m reflects repeated changes during the freezing of the deposit. The mean annual temperature of frozen rocks is −1.0°C. From 2016, mean annual ground temperatures have been observed to be gradually increasing at a depth of 1.0 m, which correlates with a stable, positive trend of the mean annual air temperatures on the Oka Plateau over the past 60 years. The species and quantitative composition of the palynospectra of the permafrost provides the evidence of repeated changes in the habitat area of forests, as well as changes in the sedimentation regime in the Holocene. According to 14C dating of the soil horizons in the upper parts of the frost mounds, after the emptying of the paleo‐lake, organic material intensely accumulated in the drained areas (from 0.5 to 0.2 ka) and ice‐mineral frost mounds began to form.